Any suitable humidity sensors for +125 degree C environment

Hi BigG : ) Interesting use-case : )

I wonder if (say) you have a small chamber with holes, and some cloth inside it, and a deliberate heater (say a PCB with pattern etched), which turned on (say) every five minutes, and heated the cloth. If the cloth was wet, then it would give off steam, which would cause some PCB with meshing 'tongues' design to conduct due to condensation. It would only need to turn on the heater if it wasn't already conducting, so perhaps the battery could still last long enough. I think you've got a great application for a commercial product..

I found a couple of references:

https://www.vsl.nl/en/about-vsl/news/relative-humidity-above-100-%C2%B0c

http://www.tandfonline.com/doi/abs/10.1080/07373938308916773?journalCode=ldrt20

It doesn't look like an easy thing to do, and certainly not low cost - why not remove some air, cool it to a more reasonable temperature, measure the humidity and estimate the original from that.

MK

These Honeywell ones state operating to 125C. Storage temp range is same.

https://sensing.honeywell.com/hih9000-datasheet-009076-7-en.pdf

You lose the in-built temperature compensation above 50C.

Read the bit about the recovery time after being through a reflow process (presumably, you'd see a lesser form of that if you took it up to 150C).

Thanks for the feedback so far. Confirms so far that nothing immediately obvious / widely known... but you never know.

The only one I could find listed on Farnell website is an IST-AG sensor for oil applications that can cope with temperatures up to 190 degree, but rather expensive.

https://www.ist-ag.com/sites/default/files/DHMK33-W_E.pdf

@shabaz, yes I suspect it may well require some inventive thinking. The purpose is monitor change in humidity to provide a more optimum determination of when to stop drying as currently this would be a timed process. Data from device will be wirelessly transmitted to a hub real time during drying operation. So key is having something that produces repeatable results for comparison rather than worrying about absolute values.

Found this:

relatv_c - Mac Instruments

Had found this useful reference - just adding for others. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4063076/

I'm holding out that there may be an off-the-shelf sensor somewhere, but I'm now thinking this is more akin to a design challenge where the aim is to detect the relative amount of steam in the air. Instead of dust particles, for example, you want something that can somehow detect "hyped up" water molecules...

While I wait for feedback, I'm now in the realms of bluesky thinking.

One whacky idea is to somehow incorporate @shabaz's idea (e.g. a small chamber with holes) with a white led (as light source) and a RGB light sensor (e.g ISL29125 light sensor (Sparkfun breakout)). Hmmm, I wonder if that would work?

I have now found a couple that claim to operate up to 150 degree C.

https://www.ist-ag.com/sites/default/files/DHP14-FemtoCap_E.pdf

https://shop.bb-sensors.com/out/media/Datasheet_Capacitive%20humdidty%20sensor_KFS140_TO_new.pdf

https://shop.bb-sensors.com/out/media/Datasheet_Capacitive_humidity_sensor_KFS140-D.pdf

I think it will be worthing testing these, but first I need to understand how to design a suitable electronic circuit for accurate measurement at high temperatures. I note that signal waveform is AC. Any tips...

I think that's a great idea. I don't know why I didn't think of it before, I remember noticing that my home-made dust sensor was also sensitive to humidity, when I was road-testing a TI capacitance-to-digital converter. But that won't operate beyond 125 degrees Celsius.

With the sensors you have found, a possible procedure could be to build an oscillator in conjunction with a inductor and some active device like a FET. Then the frequency would change as the humidity changes. But it would be a high frequency, so to measure that frequency perhaps a counter based on a CPLD could be used. Maybe there is some other counter IC out there. Or you could transmit the oscillator signal at low power and pick it up outside the dryer, but I suspect that is no good because you likely want this to work where there are many dryers nearby too.

Another wacky idea would be to send a tight beam of microwaves through the chamber. Since water vapour absorbs microwave energy, the resulting receive signal strength would give a measure of the humidity. Maybe. Would have the advantage that you wouldn't then need to insulate your electronics from the high temperature (which I think you're going to find a bit of a challenge).